Effect of molybdenum on continuous cooling bainite transformation of low-carbon microalloyed steel

Through simulation of thermomechanical processing/on-line accelerated cooling processing and observation of microstructure, the effect of molybdenum on continuous cooling bainite transformation of ultra-low carbon microalloyed steel was studied. The continuous cooling transformation curves of the trial steels with or without molybdenum addition were also determined. The result showed that the separate temperature of bainite was obviously reduced and the size of microstructure became smaller as 0.40 wt% Mo was added to the steel. At the same time, the martensitic structure, which formed at some cooling conditions, became finer and dispersed more uniformly. The deformed austenite would transform to finer bainite even when the cooling rate was not too high.     

Bainite transformation during continuous cooling of low carbon microalloyed steel

Abstract

The evolution of the final microstructure for a low carbon Nb–Ti microalloyed plate steel was studied during a simulation of thermomechanical processing for hot rolling following by accelerated cooling. The effects of austenite deformation below the non-recrystallisation temperature T _NR, cooling rate, and interrupt temperature on the formation of conventional (intergranular) bainite (CB), acicular ferrite (intragranular) (AF), and martensite–austenite (MA) constituents were determined. With increases in austenite deformation and cooling rate, and decrease in the interrupt temperature, the final microstructure changed from a mixture of CB+MA through CB+AF+MA to a dual phase AF+MA.     

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

Abstract

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (～20 μm) and a coarse average grain size (～260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ～534°C.     

奥氏体变形对铌微合金钢贝氏体相变的影响

用Gleeb le-1500热模拟机研究了铌微合金钢奥氏体变形后连续冷却条件下贝氏体相变规律.研究表明,随变形温度的升高,先共析铁素体量较少,贝氏体的板条束变宽.在连续冷却条件下,贝氏体的转变量随变形量的增加而减少,随应变速率的增加而减少,但应变速率对贝氏体转变量的影响随冷却速度的增加而减弱.随着变形后保温时间的延长,奥氏体稳定性增加,在较快冷却条件下转变产物中存在残余奥氏体.     

V-Ti钢热变形奥氏体的连续冷却转变行为

为研究V—Ti微合金钢热变形奥氏体的连续冷却转变行为,在对V—Ti徽合金钷进行Gleeble3800热模拟后,建立了连续冷却转变曲线（CCT图）,探讨了变形量和高温停留对CCT曲线和相变组织的影响．研究表明：变形量的增大,促进了先共析铁素体和珠光体转变,使其孕育期缩短,一定程度上也促进了高冷速下的贝氏体转变,但使低冷速下的贝氏体转变受阻;马氏体转变温度的降低说明变形量的增大在一定程度上使马氏体相变受阻;变形后若高温停留,组织发生静态回复,对扩散型相变的先共析铁素体和珠光体转变以及半扩散型相变的贝氏体转变均不利,使先共析铁素体转变量和珠光体转变量都有相对减少,而对马氏体转变影响不大．     

Effect of austenite microstructure and cooling rate on transformation characteristics in a low carbon Nb-V microalloyed steel

Deformation dilatometry has been used to simulate controlled hot rolling followed by cooling of a Nb-V low carbon steel, looking for conditions corresponding to wide austenite grain size distributions prior to transformation. Recrystallization and non-recrystallization deformation schedules were applied, followed by controlled cooling at rates from 0.1 °C/s to about 200 °C/s, and the corresponding continuous cooling transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling rates to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformations, displacing the CCT curve to higher temperatures and shorter times. However, it was found that the accelerating effect of strain on bainite transformation weakened as the cooling rate diminished and the polygonal ferrite formation was enhanced. Moreover, it was found that plastic deformation had different effects on the refinement of the microstructure, depending on the cooling rate. An analysis of the microstructural heterogeneities that can impair toughness behavior has been done.     

Effect of heating rate on reaustenitisation of low carbon niobium microalloyed steel

Abstract

Austenite formation during a continuous heating in a low carbon niobium microalloyed steel with a pearlite and ferrite initial microstructure has been studied. Characteristic transformation temperatures, Ac ₁, Ac _θ and Ac ₃ and the evolution of austenite formation have been determined by combining dilatometry and metallography in a range of heating rates from 0˙05 to 10 K s^–1. It has been observed that nucleation and growth of austenite depends highly on the applied heating rate. At low heating rates (0˙05 K s^–1) nucleation of austenite takes place both at pearlite nodules and at ferrite grain boundaries, while for higher heating rates (≥0˙5 K s^–1), nucleation at grain boundaries is barely present compared to the nucleation at pearlite nodules. The heating rate also affects the austenite growth path and morphology and, thus, the distribution of martensite in the dual phase microstructure obtained at room temperature.     

Investigation on decomposition behavior of austenite under continuous cooling in vanadium microalloyed steel (30MSV6)

In the present study, investigations are focused on microstructural evolution and the resulting hardness during continuous cooling transformation (CCT) in a commercial vanadium microalloyed steel (30MSV6). Furthermore, the effects of cooling rate and austenite grain size (AGS) on CCT behavior of the steel have been studied by employing high-resolution dilatometry. Quantitative metallography accompanied with scanning electron microscopy (SEM) has efficiently confirmed the dilatometric measurements of transformation kinetics and austenite decomposition products. A semi-empirical model has been proposed for prediction of microstructural development during austenite decomposition of the steel and the resultant hardness. The model consists of 8 sub-models including ferrite transformation start temperature, ferrite growth, pearlite start temperature, pearlite growth, bainite start temperature, bainite growth, martensite start temperature and hardness. The transformed fractions of ferrite, pearlite and bainite have been described using semi-empirical Johnson–Mehl–Avrami–Kolmogorov (JMAK) approach in combination with Scheil's equation of additivity. The JMAK rate parameter for bainite has been formulated using a diffusion-controlled model. Predictions of the proposed model were found to be in close agreement with the experimental measurements.     

A study on austenite decomposition during continuous cooling of a low carbon steel

In this work, a model based on the finite element method and assumption of second order phase transformation has been developed to predict temperature history and austenite decomposition kinetics during continuous cooling of a low carbon steel. In order to accurately assess the temperature field and transformation rate the effects of various factors such as work hardening role on the kinetics of transformation, interconnection between austenite phase change and thermo-physical properties of the steel, and initial austenite grain size have been considered in the model. To verify the results of the modelling, time-temperature histories during cooling of a low carbon steel has been determined and microstructural studies have been performed. The comparison between the predictions and the experimental results indicates reliability of the proposed model.     

齿轮钢20CrMoNb奥氏体晶粒尺寸及相变行为

在Gleeble1500热模拟试验机上对20CrMoNb齿轮钢进行了不同温度处理以及连续冷却相变实验,使用光学显微镜、透射电子显微镜以及膨胀曲线方法研究了齿轮钢20CrMoNb加热时奥氏体晶粒尺寸变化及连续冷却相变行为.实验结果表明,加热温度在1050℃以下时,奥氏体晶粒细小;超过此温度,NbC粒子数量因溶解而大大降低,对晶界的钉扎作用消失,奥氏体晶粒急剧粗化.20CrMoNb齿轮钢含有一定量的Mo、Nb元素,奥氏体比较稳定,出现先共析铁素体与珠光体的冷速很小.     

Effect of austenisation temperature on bainite transformation below martensite starting temperature

Effects of austenisation temperature on martensite and bainite transformation behaviour, microstructure, and mechanical properties of a bainitic steel austempered below martensite starting temperature were investigated in this study. Results show that the amount of athermal martensite gradually increased with the increase of austenisation temperature, whereas the amounts of bainite and retained austenite initially increased and then decreased, resulting in the trend of the first increase and then decrease in the product of tensile strength and elongation. In addition, the transformation rate of isothermal bainite after athermal martensite formation revealed a trend of deceleration and then acceleration with austenisation temperature at the beginning period. Moreover, the size of bainite plates decreased first and then increased with austenisation temperature.     

Continuous cooling transformation behaviour and bainite formation kinetics of new bainitic steel

In order to avoid the appearance of soft particles composed of ferrite or pearlite in the actual production of new bainitic steel, the phase transformation behaviour and bainite formation kinetics were investigated by DIL805A dilatometer, optical microscopy, scanning electron microscopy and Vickers-durometer. The results show that the soft particles cannot appear when the cooling rate exceeds 0.025?K?s^?1, and this condition can be ensured by direct spray cooling in production. The local activation energy decreases with increasing transformed bainite volume fraction (f_b), and the average energy is about 136.7?kJ?mol^?1. The local Avrami exponent mainly lies between 0.5 and 3 in a wide f_b range, indicating that the dominating mechanism of bainite formation is two-dimensional and one-dimensional growth.     

Incomplete transformation of upper bainite in Nb bearing low carbon steels

Abstract

Kinetics and microstructure of bainite transformation in Fe–(0·15 or 0·05)C–0·2Si–1·5Mn (mass%) alloys with Nb addition of 0·03 mass%. Bainite transformation occurs at temperatures below 873 K. At 853 K, transformation rapidly proceeds by formation of bainitic ferrite without carbide precipitation, but transformation stasis appears for a certain period in the Nb added alloys leaving untransformed austenite film between neighbouring bainitic ferrites. On the other band, the Nb free alloys do not show such a stasis until the transformation is completed. By further holding, the transformation in the Nb added alloy restarts by forming the mixture of dislocation free ferrite with cementite precipitation in the austenite films. In contrast, bainite transformation accompanying cementite precipitation occurs in both Nb free and Nb added alloys at 773 K, resulting in no difference in transformation kinetics. It is proposed that the incomplete transformation is caused by suppression of ferrite nucleation at interphase boundaries between pre-existing bainitic ferrite and austenite due to Nb segregation.     

Influence of molybdenum content on transformation behavior of high performance bridge steel during continuous cooling

The continuous-cooling-transformation (CCT) diagrams of high performance bridge steel with different molybdenum content were plotted by means of a combined method of dilatometry and metallography. The results show that the molybdenum addition of 0.17 wt% does not noticeably alter the transformation behavior, whereas 0.38 wt% significantly. In addition, the molybdenum addition of 0.38 wt% completely eliminates the formation of polygonal ferrite (PF) and significantly lower the granular ferrite (GF) transformation starting temperatures throughout the range of cooling rates studied. At lower cooling rates, with the increase of the molybdenum content, the martensite/austenite (M/A) constituents are noticeably refined, whereas the effects are not obvious at higher cooling rates. Moreover, the molybdenum addition of 0.38 wt% can significantly increase the Vickers hardness, but the Vickers hardness increments (by comparison of Mo-0.17wt% steel and Mo-0.38wt% steel) are sharply reduced at the cooling rate of 30 °C/s, indicating that at higher cooling rate, the molybdenum usage can be saved and the higher strengthen can be also gained. It could be found the GF transformation starting temperature is linear with the cooling rate. The empirical equation was established to calculate GF transformation starting temperatures, and the calculated values are in good agreement with measured ones.     

Acoustic emission monitoring of bainitic and martensitic transformation in medium carbon steel during continuous cooling

Abstract

This paper concerns acoustic emission (AE) measurements during continuous cooling of steel C45 using a Gleeble 1500 thermomechanical simulator. After austenising at a certain temperature, the studied specimen was cooled down and the root mean square (RMS) value of the continuous AE signal was measured. During cooling two distinct peaks in the RMS data were observed at temperatures of 200-300°C and 500-600°C, which have been attributed to martensite and bainite formation respectively. The observed bainite peak strongly indicated that the mechanism of bainite growth is displacive. The AE monitoring of bainite and martensite formation was supported by dilatation measurements, which were performed simultaneously. The effect of the austenite grain size on the evolution of the bainitic and martensitic transformation was studied by varying the austenising temperature T _a. It was found that upon lowering T _a, i.e. with decreasing austenite grain size, the bainite peak increases while the martensite peak decreases.     

奥氏体变形对22CrSH齿轮钢连续冷却相变的影响

为了研究奥氏体变形对22CrSH齿轮钢连续冷却相变的影响,在Gleeble 1500热模拟机上,将22CrSH钢在950℃变形0.4及未变形处理,然后连续冷却.利用膨胀曲线、光学显微镜、透射显微镜,结合各种腐蚀方法,分析了22CrSH钢相变行为及相变组织.研究表明:奥氏体变形使多边形铁素体加珠光体混合组织的临界冷速增大;当奥氏体变形及降低冷速时,大量的晶界仿晶型铁素体占据了奥氏体晶界,贝氏体相变向针状铁素体相变转变;变形使奥氏体在中温相变区稳定性增加,室温组织中M/A岛的数量增多.     

The intermediate transformation of Mn-Mo-Nb steel during continuous cooling

The continuous cooling transformation diagram for low-carbon low-alloy steel containing 0.05% C, 1.99% Mn, 0.31% Mo and 0.06% Nb was constructed by dilatometry and metallography. The intermediate transformation between martensite and polygonal ferrite involves two typical stages: the formation of ferrite matrix and the formation of microphases. Four intermediate transformation products obtained from various cooling rates and designated B₁, B₂, A₁ and A₂, were studied. The B₁ and B₂ structures are composed of pockets of parallel ferrite laths and interlath microphases, which are films of retained austenite in B, and are fragments of retained austenite or martensite or martensite-retained austenite (M-A) constituents in B₂. The B₁ structure is further characterized by the appearance of martensite particles inside the ferrite laths. The A₁ structure is comprised of the randomly arranged ferrite groups. Each group contains several short ferrite laths in the same crystallographic orientation and granular M-A constituents or martensite located at the rim of ferrite laths or groups. The A₂ structure is morphologically analogous to Widmanstätten ferrite. The formation mechanisms of these products are also discussed.     

Carbide-free bainite in medium carbon steel

Heat-treatment processes to obtain carbide-free upper bainite, low bainite and low-temperature bainite in the 34MnSiCrAlNiMo medium-carbon steel were explored. Results show that in the steel bainite transformation mainly goes through three stages: short incubation, explosive nucleation and slow growth. When transformation temperature, T > Ms + 75 °C, upper bainite consisted of catenary bainitic ferrite and blocky retained austenite is obtained in the steel. When Ms + 10 °C < T < Ms + 75 °C, lower bainite is the main morphology composed of lath-like bainitic ferrite and flake-like retained austenite. When T < Ms + 10 °C, the lower bainite, also known as low-temperature bainite, is obtained, which contains much thinner lath-like bainitic ferrite and film-like retained austenite. Mechanical testing results show that the lower the transformation temperature is, the better comprehensive performance is. The low-temperature bainite has the very high tensile strength and impact toughness simultaneously. The lower bainite has lower tensile strength and higher impact toughness. The upper bainite has higher tensile strength and lower impact toughness. The big difference of the mechanical performance between these kinds of bainite is mainly caused by interface morphology, size, and phase interface structure of the bainitic ferrite and the retained austenite. Additionally, when the bainite transformation temperature is decreased, the high-angle misorientation fraction in packets of bainite ferrite plates is increased. High-angle misorientation between phase interfaces can prevent crack propagation, and thus improves impact toughness.